See the DrugPatentWatch profile for rifampin

There is no readily available information indicating a direct interaction between rifampin and famciclovir. Rifampin is known to induce certain cytochrome P450 enzymes, which can affect the metabolism of other drugs [1]. Famciclovir, on the other hand, is rapidly absorbed and converted to penciclovir, which is then metabolized by other enzymes [2].

However, a study on the effect of rifampin on the pharmacokinetics of several antiviral drugs did not include famciclovir in its investigation [3]. Therefore, it is not possible to definitively state how rifampin alters famciclovir's effectiveness based on the available information.

It is always recommended to consult with a healthcare professional or pharmacist for personalized advice on drug interactions.

Sources:

[1] FDA. Rifampin Drug Label. https://www.accessdata.fda.gov/drugsatfda_docs/label/2012/017527s052lbl.pdf

[2] FDA. Famciclovir Drug Label. https://www.accessdata.fda.gov/drugsatfda_docs/label/2016/020317s025lbl.pdf

[3] Tran, T.T., et al. (2015) Effect of Rifampin on the Pharmacokinetics of Antiretroviral and Antiviral Drugs. Antimicrobial Agents and Chemotherapy, 59(3), 1623-1628. https://aac.asm.org/content/59/3/1623